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Old 11th November 2008, 01:19 PM   #1
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Default Yet another volume pot wiring question

Here's my setup

PC source > SPDIF > DCX2496 > Quad gang pot (Alps 20K Log)> 41Hz Amp9 basic

Now, right now I'm only using 3 of the gangs as I've got a 2.1 speaker setup.

Now, it's all working fine, but the only problem is the pot is very very sensitive! I only have to move it a small amount and it goes from very quiet to very loud!

Now, I was wondering if I could add some fixed value resistors inline with my volume pot in order to give less gain?

What if I added a 20k inline with the signal input before it goes to the pot?
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Old 11th November 2008, 08:40 PM   #2
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Hello Mike, You can wire another resistor in series with the input to your currrent volume control, but you will affect your input impedance. Best to try it out and see what happens. You could also put in a voltage divider in front, this would help keep the input resistance a little closer to constant, but it will still vary when you change the pot position. You could even put a variable pot wired inline with your current pot and adjust this until you find a good position. Then you leave that pot alone and use your current one as your volume control. To make the pot into a variable resistor inline, just wire your input to one of the legs, your volume control to the other leg, and then attach the wiper to one of the legs of the fixed portion of the resistor.

Peace,

Dave
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Old 13th November 2008, 12:14 AM   #3
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Nice idea with the extra pot to find the needed resistance, I'll give it a go. Once I find the needed resistance I'll just measure it with a multimeter, then replace the pot with the equivalent fixed resistor on all the channels.

I'm not 100% sure if changing the input impedance will be a problem, I think the the DCX2496 uses a simple(ish) op-amp buffer circuit on it's analogue outputs so I think I can get away with changing the input impedance without causing any problems (FWIW, the DCX2496 manual states that the output impedance it 160Ohm at 1KHz, yep it sounds fine even tho I'm using a 20k Log pot right now).
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Old 13th November 2008, 12:48 AM   #4
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Hey Mike, You're probably right about the input impedance variation not being a problem. Just wanted to mention it so you could consider it. The second pot idea just sort of came to me. Please report back about the results.

Peace,

Dave
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Old 13th November 2008, 06:51 AM   #5
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Are you sure you have the pot wired right?

The sudden increase in volume sounds to me like you've got your wires crossed. (BTDT)

EDIT: Of course the stock DCX has a very "hot" output. Much higher than the typical CD player. That could be part of the problem
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Old 13th November 2008, 09:57 PM   #6
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Quote:
Originally posted by dave_gerecke
Hey Mike, You're probably right about the input impedance variation not being a problem. Just wanted to mention it so you could consider it. The second pot idea just sort of came to me. Please report back about the results.

Peace,

Dave
Thanks. I must admit I don't really understand input impedance too well at the moment, but I have no problems with output (speaker) impedance, series/parallel wiring, etc...

I was gonna give it go tonight, but I feel too tired and will probably end up melting something I shouldn't with the soldering iron

I've got tomorrow off work so I'll give it a go then, I'm expecting a delivery of a DEQ2496 too but hopefully that won't sidetrack me too much.
Quote:
Originally posted by panomaniac
Are you sure you have the pot wired right?

The sudden increase in volume sounds to me like you've got your wires crossed. (BTDT)

EDIT: Of course the stock DCX has a very "hot" output. Much higher than the typical CD player. That could be part of the problem
I've checked my pot wiring and it's fine, I think it's the DCX giving out lots of voltage, it is intended for pro-sound use after all. I don't really want to turn down the levels on the DCX as I will lose resolution on the input DAC (I won't be getting the full 24-bit with the internal gain reduced), so i think the resistor method should work.
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Old 13th November 2008, 10:24 PM   #7
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Mike, a few questions and comments.

1) Are you sure the pot you are using is Log or audio taper?

2) putting a resistor in series is not a good idea.

3) using a couple of resistors as a voltage divider has some merit, however remember that your systems is what is loosely called a "passive pre-amp" so you have various things in conflict (eg. needing to have low Z source "looking" at a high Z load). In your case the pot is presenting a 20k load to the behringer (which is probably has a Z in the 100s). But it is also presenting a 5 k output to your amp's input Z (pot value divided by 4, in its worst case). The trick is to try and get substantial output to input mismatches (low Z to high Z).

4) Keeping #3 in mind, what you want to do is change the taper by using a shunt resistor in combo with the pot. Please do a search on "shunt" + "passive pre-amp" to get more info.

Incidentally a problem with ganged pots is that they can have mismatched values or not track identically. Fortunately the shunt resistor will also help with that problem to a certain extent.

Good Luck
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Old 14th November 2008, 05:26 PM   #8
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Quote:
Originally posted by WithTarragon
1) Are you sure the pot you are using is Log or audio taper?
I've managed to dig out the Alps datasheet (the model number RK27114), and going by the number on the pot it actually 10K log. Alps don't appear to do this pot with Linear taper AKAIK.

I also tried using a multimeter set to resistance mode connected to the output rca, and found it does appear to be log taper.
Quote:
Originally posted by WithTarragon
3) using a couple of resistors as a voltage divider has some merit, however remember that your systems is what is loosely called a "passive pre-amp" so you have various things in conflict (eg. needing to have low Z source "looking" at a high Z load). In your case the pot is presenting a 20k load to the behringer (which is probably has a Z in the 100s). But it is also presenting a 5 k output to your amp's input Z (pot value divided by 4, in its worst case). The trick is to try and get substantial output to input mismatches (low Z to high Z).

4) Keeping #3 in mind, what you want to do is change the taper by using a shunt resistor in combo with the pot. Please do a search on "shunt" + "passive pre-amp" to get more info.

Incidentally a problem with ganged pots is that they can have mismatched values or not track identically. Fortunately the shunt resistor will also help with that problem to a certain extent.

Good Luck
Thanks. In the datasheet is actually says that there is a 10% difference between gangs so I was hoping that if I made the pot less sensitive then I would reduce the output difference between channel somewhat.

I do eventually plan to measure the voltage on each of the channels with some sine waves, and I can adjust the gain output on each channel of the DCX to try and get them roughly equal.

I'll read up on passive pre-amp's and shunt resistors, by the sounds of it this is the best way of doing things.
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